Automatic dynamic focusing for computed tomography

ABSTRACT

A dynamic focusing device is provided for an X-ray scanner, the device comprising at least one detector module located in a position to receive, be illuminated by, and respond to X-rays. A source of X-rays is movable toward or away from the detector module. The module comprises a plurality of crystals each having a scintillation surface located in a common plane, with a plurality of septa separating the crystals. The septa have a height which is upstanding above the surfaces of the crystals far enough to reduce lateral X-ray scatter and to cast a shadow upon the surfaces of the crystals responsive to an illumination thereof from said X-ray source. The invention dynamically positions the detector module relative to the distance between the detector module and the source of X-rays in order to reduce substantially to a minimum any shadow in the X-rays cast by the septa upon the surfaces of the crystals.

This application is a continuation of prior application Ser. No.07/122,797, filed 11/19/87.

This invention relates to computed tomography (commonly called "CAT orCT scanning") and more particularly to means and methods for dynamicallyfocusing or directing detectors for CAT scanners directly toward asource of X-rays.

Reference is made to my simultaneously filed and copending patentapplications Ser. Nos. 07/122,909, filed 11/19/87, entitled"Cam-Controlled Automatic Dynamic Focusing For Computed Tomography", nowU.S. Pat. No. 4,872,191, and No. 07/122,905 entitled "Blinder for CATScanner", now abandoned, a continuation-in-part of which is now U.S.Pat. No. 4,891,833, for a further description of some of the elementsdisclosed herein.

A single-channel, discrete, solid-state X-ray detector has an individualscintillation crystal mounted on a dedicated silicon photodiode. Amultiplicity of such detectors may be joined together for use as amulti-channel module. Such a module may be fabricated as aone-dimensional linear array with a common multi-channel silicon carrierand with individual scintillation crystals separated from each other byupstanding collimating walls or septa. The linear array of detectors maybe abutted against each other in a straight line configuration; or, inthe alternative, they may be arranged in a curvilinear configuration sothat each module is radially focused on (directed toward) a common X-raysource. Imaging applications for either the straight or curvilineararrays are found in digital radiography and in computed tomography ("CATscanning").

The collimating walls or septa are positioned between the crystals togive channel-to-channel crystal separation, in order to improve X-rayscatter rejection (i.e. to prevent X-rays which are scattered laterallyacross the surfaces of the crystals from causing false readings). Thesepta are somewhat similar to the dividers along one row of an egg crateor ice cube tray. The detector crystals are buried deep within and atthe bottom of the space between the dividers.

The best scatter rejection is achieved by use of septa made from anexcellent X-ray attenuator material, such as tungsten, and by increasingthe height of the septa above the scintillation surface of the crystal.Such a height increase produces an increase in scatter rejection, sincethe rejection is proportional to the ratio of the septa height above thecrystal surface, to the scintillator width.

However, polar response, the ability of a detector to accept direct(non-scattered) x-rays from a source which is not located directly onthe focal axis of the detector, is also a function of the ratio of theheight of the septa to the active width of the individual crystals, andis inversely proportional to septa height. Thus, an increase in polarresponse requires lower septa for a given crystal width, but such alowering of the septa increases the susceptibility to false readingsfrom X-ray scattering. This forces a trade-off between polar responseand scatter rejection.

Therefore, it becomes important to control the X-ray angle of incidenceilluminating the surface of the scintillation crystal. An X-ray sourceis somewhat like a floodlight illuminating the surface of the array. Thesepta rise to a height which casts shadows upon the surfaces of thecrystals, unless the entire array is pointed directly toward the source.When the septa cast shadows on the surfaces of the crystals, it greatlyreduces the efficiency of the entire array by reducing the scintillationresponse of the crystals. This invention is designed to either eliminateor minimize such shadows by continuously and dynamically directing(focussing) the array toward the X-ray source throughout an entire CATscan.

In the past, digital radiography and computed tomography systems havehad a fixed X-ray source-to-detector distance which has only required afixed focus. Therefore, once the detectors were properly directed, therewas no need to redirect them. Recently, however, CAT scanners havevaried the source-to-detector distance for providing optimal CATscanning over a wide variety of object sizes, which is especiallyimportant for industrial uses. The newer systems are also designed todecrease X-ray scatter, and they thereby reduce the polar response ofthe detector because they employ tall tungsten septa between thescintillation crystals. In such an environment the invention mustdynamically focus the arrays of solid-state X-ray detectors in responseto a decrease in detector polar response resulting from a variation insource-to-detector distance. That is, the invention must always pointthe detector array in a direction such that the septa cast a minimumx-ray shadow.

In keeping with an aspect of the invention, a CAT scanner system isprovided for dynamically X-raying an object, such as an industrialproduct. An X-ray source is directed to illuminate a given area, whichis remote from the X-ray source by a variable distance. A detectorcomprises a plurality of crystals separated by septa for collectingX-rays falling on a surface in the given area. The septa produce shadowswhich increase or decrease as a function of the angle of incidence ofX-rays illuminating the surface of the crystals. Responsive to changesin the distance between the X-ray source and detector, the detector ispositioned in a manner to adjust the angle of incidence for reducing theshadow to substantially a minimum amount.

A preferred embodiment of the invention is shown in the attacheddrawings, wherein:

FIG. 1 pictorially shows the invention used in a CAT scanner which isabout to X-ray a rocket motor;

FIGS. 2A and B schematically show top and side views, respectively, of aCAT scanner utilizing the invention;

FIG. 3 shows a cross-section of multiple detector modules butted to forma linear array, taken along line 2--2 of FIG. 4;

FIG. 4 schematically shows a single X-ray detector module; and

FIG. 5 schematically shows a system for focusing the detector module ofFIG. 4.

FIG. 1 pictorially indicates the inventive CAT scanner as it is beingused to X-ray a rocket motor. A frame 19 supports a source 20 of X-rays,such as a 150-420 kv tube or a 2 MV linear accelerator, and an opposedX-ray detector 22 for movement to any points in a pair of spacedparallel vertical planes. The object 23 under test (here, a rocketmotor) is mounted on a turntable 24 which rotates the rocket motor (seearrows -7) by way of drive motor M7. The turntable 24 is mounted on acarriage which travels linearly over a track 25 by way of drive motorM6. Thus, the rocket motor 23 exposes all of its surfaces to X-rays asit both rotates and travels between the source 20 and the detector 22. Acomputer responds to the resulting signals from the detector 22 toconstruct a tomographic X-ray image of object 23.

A number of separate motors M1-M5 drive components mounted on thesupporting frame structure 19 to position the X-ray source 20 and thedetector 22 relative to the object 23 under test.

The operation of the CAT scanner of FIG. 1 is explained with the help ofFIG. 2 in which FIG. 2A is a stylized top view and FIG. 2B is acorresponding side view of the structure shown in FIG. 1. The source 20of X-rays is displaced from the detector 22 by a source-to-detectordistance 26, which may vary. The X-rays tend to spread into a somewhatfan-shaped pattern, as seen at 27. Pre- and post-collimator beam shapers28, 30 are positioned within the path of the X-rays to reduce the widthor thickness of this fan-shaped pattern of X-rays. A pair of plates 32made from material which is opaque to X-rays form a blinder whichdefines a slot length 34 through which X-rays may pass in order toilluminate the surface of detector 22, which includes an array ofscintillation crystals.

The object 23 under test or study moves past the blinders 32 and acrossthe slot 34 in order to cast a shadow in the X-rays illuminating thescintillating surface of the crystals forming detector 22. They give anoutput signal in the form of the object under study, which representsthe desired X-ray image.

From an inspection of FIG. 2, it is obvious that as the X-raysource-to-detector distance 26 increases or decreases, the base of theconical X-ray beam 27 spreads or contracts within the imaging area,thereby changing the angle of incidence upon the illuminated surface ofthe detector 22.

The detector crystals are arranged in a plurality of elongated strips 36which are pivotally connected at one end 38 (FIG. 2B) to the machine andat the other end to a cam follower pin 40. The cam follower pin rides inindividually associated cam slots 64 (FIG. 4) formed in a focuser plate42. As a focusing motor 44 drives the focuser plate 42 back and forth indirections C or D, the cam-controlled end of the strip 36 of detectorcrystals swings into a position where septa on the strip 36 cast theminimum shadow on the crystal.

FIG. 3 is an enlarged cross-section of an X-ray detector 22, also shownin FIG. 4, which includes a plurality of spaced parallel septa 52extending forward from the scintillating surfaces 53 of the crystals. Anindividual scintillation crystal 54 is buried between each pair of thelongitudinal septa 52. The septa may have any suitable height, such asan inch or so above the surface 53 of the crystal. Thus, the fullsurfaces 53 of all of the crystals are maximally illuminated by theX-rays only when the detector is pointed directly toward the source ofX-rays. In that way the total effect of all shadows can be minimized.

If the source 20 of X-rays is subsequently moved to any other locationrelative to the septa, they again cast shadows over some portion of thetotal crystal surface. Thus, to aim or focus the detector, it is mountedon a mechanical support which pivots or swings about the fixed point 38.More particularly, FIG. 3 shows a corridor width 58 extending linearlyfrom a neighboring septum. When the detector is correctly positioned,this relatively narrow corridor 58 contains both the crystal and theX-ray source located perpendicularly above the crystal. When the X-raysource is properly positioned at the point "Y" within the corridor 58(for all crystals), the angle of incidence of X-rays illuminating thesurfaces 53 of the detector crystals 54 is approximately 90°. On theother hand, if the X-ray source is at point "X", for example, the angleof the incidence is too great and the detector strip 36 should pivot indirection A so that point X falls within the corridor 58. If the X-raysource is at point Z, the incident angle is too great in the oppositedirection and the detector 36 should pivot in direction B so that pointZ lies within the corridor 58.

FIG. 4 schematically shows an entire detector module 36 which is in theform of an elongated strip, plate, or bar having thereon linearly spacedsepta 52, with crystals 54 therebetween, and constructed as shown inFIG. 3. One end of the detector module 36 is pivotally connected at 38,to any suitable stable supporting structure (not shown), such as atable, for example. The opposite end of the strip, plate or bar 36comprises a cam-follower pin 40 engaging a suitable cam slot 64. Thus,depending upon the location of the source 20 of X-rays, that end of thedetector array 36 may swing in either of the directions A or B to orientthe septa 52 relative to the source. One way of finding and fixing theposition of cam follower pin 40 is to provide a plurality of sensingswitches distributed along the path of travel provided by the cam slot64. Another way is to detect the maximum strength of a signal from thedetector module. Still another way is for a microprocessor to memorizecertain desirable positions which correspond to the various distances 26(FIG. 2A) between source 20 and the surface of the detector crystals 54.

The source-to-detector or source-to-image-plane distance 26 (sometimescalled "SID") between the source 20 of X-rays and the detector module 36is of considerable importance. At the close extreme, the source shouldbe as close as possible to the detector in order to give a signal with amaximum strength and a minimum signal-to-noise ratio. At the farextreme, the X-ray beam flares out and this has a magnifying effect uponthe size of the image resulting from the X-ray. This magnification canbe seen in FIG. 2A where the fan 27 of X-rays becomes larger as thedistance 26 increases.

The principles schematically illustrated in FIGS. 2-4 are incorporatedinto the practical embodiment of FIG. 5 which focuses individualdetector modules 36 by means of an individual cam follower pins 40. Eachcam follower is dedicated to and fixed in an end of an individuallyassociated detector module 36. Each cam is a continuous slotted path 64formed in a plate 42, the shape of which is a function of the positionof the septa and the location of the scintillating surfaces in a givendetector module, relative to the position of the X-ray source 20.

FIG. 5 shows an array of detector modules 36, each constructed asdisclosed in FIGS. 3 and 4. There are any suitable number of modules 36. . . 36a, each of which is individually connected at one end to a frameor "ground" at a pivot point 38 . . . 38a. At its opposite end, eachdetector module 36 is connected into an individually associated cam slot64 . . . 64a, by a suitable cam follower pin 40 . . . 40a. There is aseparate detector module 36 (not shown) associated with each of theremaining cam slots. The cam slots are all formed in the focuser plate42 which is mounted to slide back and forth in directions C, D alonglinear bearings 74, 76. The angles of the cam slots 64 . . . 64a fan outso that the outermost modules 36 . . . 36a swing further than themodules which are between them.

If the focuser plate 42 is in the position shown in FIG. 5 (fullyextended in direction C), the detectors 36 . . . 36a are at a relativelyshallow angle E relative to the perpendicular (i.e. the detector modules36 . . . 36a are the closest that they can come to being parallel toeach other).

If the focuser plate 42 is moved as far as possible in direction D, thecam follower pins 40 . . . 40a of detectors 36 . . . 36a fan outwardlyand away from each other (the angle E becomes larger) as is shown by theoutward flair of slots 64 . . . 64a. Thus, by properly positioning thefocuser plate 42, the detectors are all optimally positioned relative tothe distance 26 (FIG. 2A) between the X-ray source 20 and the detectors36.

The outermost detector modules 36, 36a at the left and right extremes ofFIG. 5 must rotate the most for a given change in the SID, and thecentrally located ones must rotate the least. Accordingly, the cam slots64 at the leftmost side of plate 42 are angled progressively more to theleft as their locations get closer the left end of the plate, andconversely the cam slots at the right side are angled progressively moreto the right as their locations get closer to right end of the plate,whereas the center-most slots are angled the least. If one of themodules 36 . . . 36a is located dead center relative to the x-raysource, then it need not rotate at all, and needs no cam follower andcam slot.

Attached to the focuser plate 42 is a threaded drive nut 78, throughwhich a lead screw 80 passes. The lead screw is supported by anysuitable number of pillow blocks (one of which is seen at 82). A motor84 is coupled either directly or through a gear train, a timing belt, orthe like, to rotate the lead screw 80 and thus to drive the nut 78 andfocuser plate 42 back and forth in directions C, D, depending upon thedirection of lead screw rotation. Motor 84 may be a stepping motordriven by a computer counting a predetermined number of stepping pulsesor an encoder-controlled DC servomotor with closed-loop feedbackcontrol.

The position of the focuser plate 42 may be determined by one or morelimit switches or proximity sensors, one of which is shown at 86. In apreferred embodiment, the sensor 86 detects when the focuser plate 42 isin a home position. From this home position, the motor 84 is driven tomove the focuser plate 42 along linear bearings 74, 76 and thereby torotate the detectors apart or together. At some point along theexcursion C, D, the physical positions of the detectors 36 . . . 36arelative to the X-ray source 20 are such that a minimum amount of shadowis cast by the septa upon the scintillation surfaces 53 of crystals 54.At this point, the collective output of the detector array becomes amaximum.

If the motor 84 continues turning the feed screw 80, the detectors 36 .. . 36a continue to move. Soon, the septa again begin to cast largershadows on the crystals and the strength of the signals begin todiminish. Then, a suitable counter or memory in a controller circuit 88drives the motor backward to where the maximum strength signal occurred;or, the controller can simply hunt for the signal of maximum strength.

Preferably, the cam follower positioning has a maximum tolerance betweenthe cam slots and the cam followers to yield a maximum detectororientation angle error of less than 0.05 degrees.

As an alternative to the use of the hunting or counting type control, aplurality of sensors switches 86 may be located at discrete intervalsalong the entire excursion route of focuser plate 42. Then, any movementor change in the source-to-detector distance 26 (FIG. 2A) automaticallyenables an individually associated limit switch 86 which causes themotor to drive the focuser plate 42 to a position corresponding to theposition of that enabled switch. Thus, there may be independent, presetfocuser plate positions which correspond to specific source-to-detectordistances. In still another alternative, a position encoder may be usedin connection with a predetermined address for each source-to-detectordistance. Then, the motor 84 simply drives the focuser plate 42 to aposition corresponding to that address.

This dynamic focusing improves the quality of a digital radiography orcomputed tomography image by improving X-ray scatter immunity and thusincreasing the X-ray signal-to-scatter ratio. At the same time, itincreases the versatility of the CAT scanner by permitting variation ofthe source-to-detector distance, which provides for multiple CAT scannerreconstruction field sizes. Various size objects may then be imaged inthe same CAT scanner without sacrificing image quality. The focuserplate/cam concept provides for precise and simultaneous multiple X-raydetector array focusing responsive to operation of a drive motor and toposition feedback sensors. The accuracy of the focusing device is afunction of the precision of the cam slot array, permitting the detectormodules to be positioned to accuracies of less than 1/10th of onedegree, relative to the fixed pivot point 38.

Those who are skilled in the art will readily perceive how to modify theinvention. Therefore, the appended claims are to be construed to coverall equivalent structures which fall within the scope and spirit of theinvention.

We claim:
 1. A dynamic focusing device for a penetrating radiationscanner, said device comprising at least one detector means located in aposition to receive, be illuminated by, and respond to penetratingradiation; a source of penetrating radiation which is movable toward oraway from said detector means; said detector means comprising aplurality of individual detectors, a plurality of septa for separatingsaid individual detectors, said septa extending beyond said individualdetectors far enough to reduce lateral penetrating radiation scatter andto cast shadows upon said individual detectors responsive to anillumination thereof from said penetrating radiation source; and meansfor dynamically orienting said detector means as a function of thechanging distance from said detector means to said source of penetratingradiation in order to reduce substantially to a minimum any shadow castby said septa upon said individual detectors.
 2. The device of claim 1wherein said detector means is in the form of an elongated modulecarrying a plurality of said septa and individual detectors, saidelongated module having a pivot point at one end, and said dynamicorienting means including focuser means for swinging said module aboutsaid pivot point in response to movement of said focuser means.
 3. Thedevice of claim 2 wherein there are a plurality of said detector means,said focuser means being individually coupled to swing at least one ofsaid modules about said pivot point.
 4. The device of claim 3 whereinsaid focuser means swings said modules to orientations in which all ofsaid individual detectors are focused on said source of penetratingradiation at each of a plurality of distances between said source ofpenetrating radiation and said detector means.
 5. The device of claim 4wherein said detector means give output signals of different strengthsdepending upon the orientations to which said modules are swung by saidfocuser means, means for detecting the maximum of said output signals,and means for causing said focuser means to swing said modules to apoint which gives an output signal having a maximum signal strength. 6.The device of claim 4, wherein said detector means travels along anexcursion route, said device further comprising a plurality of sensingswitches distributed along the excursion route of said detector meansfor identifying a plurality of orientations to which said modules maymove, means for selectively enabling one of said sensing switchescorresponding to a distance between said source of penetrating radiationand said detector means, and means for moving said modules means tolocations identified by said enabled sensing switch.
 7. The device ofclaim 4 further comprising means for memorizing a plurality of positionsof said detector means relative to respective distances between saidsource of penetrating radiation and said detector means, and meansresponsive to a change in said distance between said source ofpenetrating radiation and said detector means for moving said detectormeans to a memorized orientation corresponding to the new distance ofsaid source of penetrating radiation.
 8. A system for dynamicallyscanning an object under study using penetrating radiation, said systemcomprising means for directing a penetrating radiation source toilluminate a given area which is remote from said radiation source,detector means for collecting penetrating radiation falling on a surfacein said given area, means for producing a shadow on said surface, saidshadow-producing means being interposed between said penetratingradiation source and said surface, said shadow increasing or decreasingas a function of the angle of incidence of said penetrating radiationilluminating said surface, means for changing the distance between saidpenetrating radiation source and said detector means, and meansresponsive to a change in the distance between said penetratingradiation source and said detector means for, concurrently with saidchange in distance, orienting said detector means for collectingpenetrating radiation in order to adjust said angle of incidence toreduce said shadow to substantially a minimum amount.
 9. The system ofclaim 8 wherein said shadow-producing means is at least one upstandingseptum having a height above said surface which is sufficient to controllateral scattering of penetrating radiation illuminating said surface.10. The system of claim 9 wherein said detector means comprises aplurality of detector devices which are mounted to swing between twolimits, and means for adjusting the orientations of said detectordevices in order to produce substantially a maximum detector signalstrength.